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Measurement of e + e - to multihadron cross sections using Initial State Radiation in Babar

This study presents the measurement of e+e- to multihadron cross sections using Initial State Radiation (ISR) in the Babar experiment. The cross sections for different final states, such as p+p-p0, p+p-K+K-, and 3(p+p-), are analyzed. The experiment utilizes the Babar detector at the PEP-II B-factory with a peak luminosity of 10.58 GeV. The cross section calculations take into account ISR photon emission and final state radiation correction. The results are compared with previous experiments, and systematic uncertainties are evaluated.

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Measurement of e + e - to multihadron cross sections using Initial State Radiation in Babar

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  1. Measurement of e+e- to multihadron cross sections using Initial State Radiation in Babar Matteo Negrini Frascati, Jan 19, 2006

  2. Outline • The Babar experiment • Initial state radiation (ISR) at Babar • Multihadron cross sections • e+e- p+p-p0 • e+e- 2(p+p-), p+p-K+K-, 2(K+K-) • e+e- 3(p+p-), 2(p+p-p0), 2(p+p-)K+K- • pp Disclaimer: extensive study of J/y decays in the same channels is not presented here

  3. The PEP-II B-factory 9.0 GeV e- 3.1 GeV e+ ECM=10.58 GeV Peak Luminosity: 1034 cm-2 s-1

  4. The Babar Detector SVT: 97% efficiency 20mm z resolution SVT+DCH: s(pt)/pt=0.13%·pt+0.45% EMC: s(E)/E=2.32%·E-1/4+1.85% 1m

  5. Particle Identification

  6. gISR e- hadrons s’ e+ ISR at Y(4S) energy • s of the machine fixed at 10.58 GeV • Radiation of a hard photon  s’=0-5 GeV • The gISR is detected in the analysis (acceptance ~10-15%) • Eg ~ 3-5 GeV  small fake beam gas background • The hadronic state is collimated on the recoil • Final state radiation contribution small and well separated • s’ determined from the final state • energy resolution depends on tracking • entire energy scan of the cross sections in a single “shot” • m+m- used for luminosity normalization

  7. ISR luminosity ISR cross section for the production of the final state f: x=2Eg/s s’=s(1-x) Probability of ISR photon emission:

  8. Cross section calculation dNfg = number of ISR f events dNmmg = number of ISR mm events dFSR = final state radiation correction (negligible for most f)

  9. e+e-p+p-p0 c2 distribution in the range 1.05<M3p<3 GeV/c2 Selection: • particles inside fiducial volume • Eg>3 GeV for at least one g • kinematic fit applied • fit c2 used for event selection Backgrounds: • p+p-p0p0, p+p-, K+K-p0, t+t-, ... Bkg suppression cuts (improve S/B) • no track identified as K • Ep0>0.4 GeV • kinematical fit to 4p (to reject p+p-2p0) • Mpg>1.5 GeV/c2 (to reject tt) Prel. selection (data/MC) p+p-p0p0 other bkg processes Selected Bkg/efficiency study 89 fb-1 PRD 70, 072004 (2004)

  10. e+e-p+p-p0 Inconsistent with DM2 SND Babar DM2 w’’ M=1350±28 GeV/c2 G=450±100 GeV/c2 w’ w M=1660±10 GeV/c2 G=230±36 GeV/c2 f

  11. Systematic uncertainties e = efficiency eMC = eff. measured on MC d = correction

  12. e+e- 4h p+p-p+p- K+K-p+p- K+K-K+K- Selection: • particles inside fiducial volume • Eg>3 GeV for at least one g • 4 charged tracks • kinematical fits applied • 4p always applied • 2p2K and 4K applied if K are present • fit c2 used for event selection Background: • ISR t+t- and multihadron Separation of different channels based on the c2 for the different kinematical fit 4p candidates 4p MC Non ISR bkg (JETSET) 89 fb-1 PRD 71, 052001 (2005)

  13. e+e- p+p-p+p- Signal Signal region (MC) ISR background (control region) Control region (MC) Non ISR background (JETSET) Selection efficiency vs mass (MC) <1% 3-5% 10% Syst. error due to bkg subtraction

  14. e+e- p+p-p+p- Comparison with previous experiments Measured 4p cross section Only statistical error shown

  15. e+e- p+p-p+p-

  16. e+e- p+p-K+K- c2(2p2K)<20 c2(4p)>30 c2(4K)>20 0.5% 4p contamination negligible 4K contamination • 15% systematic uncertainty • Dominated by: • 10% uncertainty in the acceptance (MC) • 5% difference data/MC in K ID Only statistical error shown

  17. e+e- p+p-K+K- 2p2K channel dominated by intermediate K*0Kp After removing events in K*0 bands f K*0(892) r “Other” Kp combination

  18. e+e- K+K-K+K- c2(4K)<20 high purity sample • 25% systematic uncertainty • Contributions: • absence of detailed model for acceptance • uncertainty in bkg subtraction • difference data/MC in K ID Only statistical error shown

  19. e+e- 6h 3(p+p-) 2(p+p-)p0p0 K+K-2(p+p-) Selection: • particles inside fiducial volume • Eg>3 GeV for at least one g • 6 tracks or 4 tracks and 4g (Eg>20 MeV) • kinematical fits applied • fit c2 used for event selection Background: • ISR t+t- and multihadron 232 fb-1

  20. e+e- 3(p+p-) Signal ISR background (control region) Signal region (MC) Non ISR background (JETSET) Control region (MC) Selection efficiency vs mass (MC) <3% 3-5% Syst. error due to bkg subtraction

  21. e+e- 3(p+p-) Intermediate state r0(770)2(p+p-) assumed in MC production All pions point = data histogram = MC

  22. e+e- 3(p+p-) r0(770)2(p+p-) model gives a very good description of the data No other significant structures observed (but full partial wave analysis not done) A 6p phase space simulation does not produce deviations in the angular distribution The acceptance varies by less than 3%, which is taken as a systematic uncertainty MC (J/y not included) Data

  23. e+e- 3(p+p-) Structure at 1.9 GeV also seen by FOCUS FOCUS collab. PLB 514, 240 (2001) Only statistical error shown

  24. e+e- 3(p+p-)

  25. e+e- 2(p+p- p0) • Additional bkg reduction cuts: • require Eg>50 MeV • K veto on tracks Structure at 1.9 GeV confirmed in 2(p+p-p0) Only statistical error shown

  26. e+e- 2(p+p- p0)

  27. e+e- 2(p+p- p0) Data MC Presence of resonant structures w r+ J/y w h w r0 f0 p+p-p0 mass “other” p+p-p0 mass Evidence of wh production

  28. e+e- 2(p+p- p0) Structure in wh channel Fit to resonance: m=1.645±0.008 GeV/c2 G=0.114±0.014 GeV/c2 peak cross section=3.08±0.33 nb 2(p+p-p0) wp+p-p0 f(1680) ? w(1650) ? wh

  29. Combining the 6p channels Fit to Breit-Wigner structure by several states decaying to the same mode Continuum Resonance m = 1.88±0.03 GeV/c2 G = 0.13±0.03 GeV f = 21±40˚ m = 1.86±0.02 GeV/c2 G = 0.16±0.02 GeV f = -3±15˚

  30. e+e- 2(p+p-)K+K- • At least one particle identified as K • Kinematical fit • c2(6p)>20 • 15% systematic uncertainty • Dominated by: • 10% uncertainty in the acceptance (MC) • 5% difference data/MC in K ID Only statistical error shown

  31. e+e- pp • Study of: • cross section (continuous spectrum from threshold, s=2mp, in a single measurement) • form factor in the time-like region C = Coulomb correction factor Allows s finite at threshold Experimental challenging because of larger 2-body backgrounds: mm, pp, KK Good PID and kinematical fit required 232 fb-1 hep-ex/0512023

  32. e+e- pp 30% signal loss ISR bkg suppression factors: 15·103 for pp 500·103 for mm 2·103 for KK Selection: • particles inside fiducial volume • Eg>3 GeV for at least one g • 2 charged tracks, both with proton ID • kinematical fit to C+C-g applied • C = e, m, p, K, p 25% signal loss ISR bkg suppression factors: 50 for pp, mm 30 for KK p K p K

  33. e+e- pp 1. ISR backgrounds Background Data ~4000 events N. of bkg events pp 5.9±2.5 KK 2.5±1.0 ee 2.5±1.0 mm <11 K p m

  34. e+e- pp Expected mpp distrib. from ppp0 events 2. pp + photons background Dominant background contribution from ppp0 Kinematical fit to ppgg performed

  35. e+e- pp Detection efficiency 10% systematic error From the difference between pure electric (GM=0) and pure magnetic (GE=0) values

  36. e+e- pp Cross section (systematic error included)

  37. e+e- pp |GE|/|GM| measurement from p angular distribution Two samples of ppg events generated: one with GE=0 the other with GM=0 Angular distributions HM(cosq*,s) and HE(cosq*,s) obtained from the simulation Fit to the distribution: • Free parameters: • A (normalization) • |GE/GM|

  38. e+e- pp Angular distributions for different s regions |GE/GM| measurement from angular distributions (stat. and syst. errors included) GM=0 GE=0

  39. e+e- pp Definition of an “effective form factor” under the assumption |GE|=|GM| Fit to asymptotic behavior At threshold

  40. Summary and perspectives • Extensive ISR analysis program in Babar • Full spectrum from threshold to ~4.5 GeV • All main hadronic channels under study • pp, KK, LL • p+p-2p0, p+p-3p0 • K+K-p0, K+K-2p0, fp0 • p+p-p0h, fh • ...

  41. BACKUP

  42. Systematcs overlook • Luminosity from mm: 3% • Background subtraction: ~1-10% • larger where the cross section is small • Acceptance from simulation: 1-3% • ~10% in some cases (model dependence) • Data/MC differences in tracking/PID: ~3-5%

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